In wireless networks, base stations typically broadcast “pilot signals.” Pilot signals are known and monitored by subscriber equipment (such as mobile units) within a cell area, and are used, for example, in power measurements and in the generation of coherent phase references. Each base station of a system transmits a respective pilot signal that enables the subscriber equipment to distinguish the transmissions of different base stations from each other. Measurements of the relative strengths of detected pilot signals allow mobile units to quickly connect to the nearest base station when they first power on, and allow them to help quickly determine the best candidate base stations during handoffs (switching of serving base station).
Another application of pilot signals is in the geolocation of mobile units. Using information on the positions of base stations and measurements of the detectable pilot signals of nearby base stations made by the mobile units, triangulation techniques can be used to determine the mobile unit's most likely position. Mobile geolocation techniques are a critical component of “Enhanced-911” and other location-based services.
In Code Division Multiple Access (CDMA) based systems and Universal Mobile Telecommunication System (UMTS) networks in particular for example, base stations continually broadcast pilot signals that are spread using a known (standardized) pseudo-random sequence. All base stations in a UMTS network use the same pilot sequence; however, pilot sequences used by base stations are offset from one another in time.
UMTS mobile units (and terminals in other Code-Division Multiple Access (CDMA) based technologies) are capable of measuring the relative phase differences between detected pilot sequences. The estimates of the relative phase difference of detectable pilot signals are then used by the Observed Time Difference of Arrival (OTDOA) geolocation method outlined in the UMTS system specifications to determine a mobile unit's position.
In implementing current geolocation methods employing triangulation of pilot signal measurements of serving and neighboring base stations, pilot signals from at least three different base stations are required to accurately estimate the position of a user. However, due to the inference-limited nature of CDMA-based systems such as UMTS, detecting pilot signals from two or more neighbor base stations is often not possible over a large portion of the cell's coverage area. It is well known, for example, that due to the near-far effect, when all base stations are transmitting at maximum power, in over ˜40% of the cell's coverage area (the region nearest the base station), only the pilot signal from the serving base station will be detectable by the mobile unit. When only a single base station is detectable, the error of the geolocation estimate is unacceptably high. This error can cause geolocation algorithms to fail to satisfy the stringent position error requirements outlined by the U.S. Federal Communications Commission—an error of less than 100 m 67% of the time, and less than 300 m 95% of the time for network-based solutions, and less than 50 m 67% of the time and less than 150 m 95% of the time for handset-based solutions.
In an attempt to correct for the near-far effect and to increase the chances of mobile units detecting two or more pilot signals from neighboring base stations, an Idle Period in DownLink (IPDL) method has been introduced in the UMTS system specification. The IPDL solution decreases system-wide interference by temporarily switching off the serving pilot signal for a period of time. This solution has significant drawbacks, however. The temporary muting of pilot signals adversely affects the performance of the downlink channel for on-going calls, increasing frame error rates. Implementing IPDL requires architectural changes to the UMTS network and complicates critical functions such as downlink power control.